well mounts would be constant, pump will not really give any definative vibration and ceraintly wouldnt be intermitent. what does you fan look like (behind the rad not in front) has it lost or loosing the edges or peices? it could be off balance. also check your tension pully, push on it and it should move with no problems, if it squeeks excessively or takes some serious muscle to push then it could be your problem, i personally had the problem just this week with the Tension pully and it gave me a slight vibration but not too noticeable more of a sound.

lastly check the brakes, see if you may or maynot have a caliper sieze (also check the parking brake even thou you said it feels like it comming from the front.)

Furious has got some good ideas. However, if your vibration is only occuring at a particular speed (55mph in this case) and not engine rpm (no fan theories) and only from cold to operating temp, then I would first suspsect that you've got something going on chasis and/or driveline related. I would look at tires first because they are the quickest and easiest to test. Under inflated tires will cause the problem you describe (especially in colder climates). Check the air pressures in your tires and make sure they are at the proper cold temp specification before you leave in the morning. If you don't have access to air at home, on your way home in the evening inflate the tires 7-10 psi over cold setting (less if in warmer climates). This should put them at cold spec in the morning. Give that a shot and see if your problem persists. If that doesn't work, then keep increasing the pressures slightly and note the changes. If nothing improves then you will probably have to take it in. (booger. I know) Let us know what happens. Unfortunately this is a hard one to accurately diagnose without actually driving or riding in the car.

I tried the increasing the tire pressure business, it didn't work. I'm taking it in soon but because it's an intermittent problem on cold mornings, I don't know if they can find the problem. We'll see.